DESCRIPTION (Applicant's abstract): Genetic instabilities (expansion and deletions of simple repeating sequences) are important in the life cycles of both prokaryotic and eukaryotic cells. This fundamental mechanism of mutagenesis has been found in mycoplasma, bacteria, yeast, mammalian cells, and in humans. In lower organisms, these genetic polymorphisms are the basis for phase variations which control the expression of genes. In humans, the expansions and deletions of simple repeating sequences are closely tied to the etiologies of cancers as well as hereditary neurological diseases. Prior work has revealed that expansions are mediated by DNA replication and repair by the slippage of the complementary strands of the repeats to form hairpin loop structures with differing relative stabilities. The principal investigator has recently demonstrated that recombination is a powerful mechanism for generating large expansions. To the extent that this work can be extrapolated to human diseases, recombination may be an important mechanism for the large expansions found in fragile X syndrome, myotonic dystrophy, and SCA8. The first Specific Aim is to elucidate the mechanisms of genetic recombination which mediate the triplet repeat sequence (TRS) expansions. Specific Aim 2 will evaluate the role of recombinational repair of double strand breaks in genetic instabilities. The third Specific Aim is to establish a genetic assay for determining the recombination frequency. Specific Aim 4 will investigate tandem duplication-based instabilities in vivo in recA- cells. In summary, the principal investigator will investigate the molecular mechanisms (replication, recombination, repair) that cause genetic instabilities in simple repeat sequences.